Curate, connect, and discover
I feel like he's formed that parental figure to his team even though they are almost all the same age they still look up to him.
And then you have Starscream with his abusive dad being the only thing he's ever known. He may have stared a bit to long whenever Optimus would show affection or simple respect and care to his team cause those concepts are out of reach for him. So when he finally goes to the autobots.
He can't fit in. But Optimus isn't beating him up to the brink of death or blaming him for something out of his control or that he isn't doing things right.
And at this point Starscream realises he was taken in by the wrong brother.
And I feel like this is a good point on why Starscream cared so much about Optimus when he joined the decepticons again. Cause he finally got a taste of a true father and he does cherish it.
And on Optimus' side he saw how his brother treated Starscream but not all of it or the fact that Starscream had never had anyone else or how young he is or was. So when he joined he started to realise these things but instead of pressuring it all out of him to talk he simply became what Starscream lacked his whole life knowing that the former wouldn't work on Starscream. He thinks action speak louder than words yet words are still needed. Starscream also has that ideology but instead in the way of longing for it. Along the way he does see Starscream as a son.
But things don't last forever, it didn't even last long enough to start.